High frequency signal generators are used more and more frequently in electric circuits. The term &lt;&lt;synthesiser&gt;&gt; is also used to describe such generators.
A large number of high frequency signal generators exist in the state of the art concerned. European Patent No. 0 595 377 discloses two types of high frequency synthesis: &lt;&lt;direct&gt;&gt; synthesis and &lt;&lt;indirect&gt;&gt; synthesis.
It will be noted that the present description concerns only generators associated with &lt;&lt;indirect&gt;&gt; synthesis. FIG. 1 of the present description shows a high frequency signal generator 20 which implements &lt;&lt;indirect&gt;&gt; synthesis as described in this document.
Generator 20 includes, connected in series, a voltage controlled oscillator O able to supply a signal Vout having a frequency comprised within a predetermined frequency range, a prescaler (a fixed microwave divider) P, two programmable dividers N and M, a phase and frequency comparator H, and a reference oscillator R formed of a crystal. Generator 20 also includes a phase-locked control loop connected between comparator H and oscillator O, this control loop including an amplifier A and a filter F. The control loop is also arranged so that the frequency of signal Vout is substantially constant and equal to a desired nominal frequency. It is to be noted that dividers N and M are suitably programmed so that the frequency of signal Vout (which is equal to (R/M)*N*P)) corresponds to the desired nominal frequency.
One problem encountered with such a generator is that the real frequency of signal Vout is substantially different to the nominal frequency, particularly because of the instability of the reference oscillator or even under the effect of the ambient temperature. As a result, the difference between the real frequency and the nominal frequency becomes greater than frequency precision required for the generator application, which can be detrimental for applications requiring high frequency precision.
By way of illustration, a generator of this type can form part of a telecommunication system using a narrow band transmission channel, for example in cellular telephones or portable radio-frequency devices. In this case, it is necessary to use a generator supplying a signal whose carrier frequency is defined with great precision, so that the data transfer occurs over a narrow frequency range. Typically, within the scope of such an application, the generator frequency precision must be of the order of several ppm (10.sup.-6).
One solution to the aforementioned problem consists in using a crystal able to supply an extremely stable and constant low frequency. An &lt;&lt;AT&gt;&gt; type crystal whose frequency precision is of the order of 2 to 10 ppm is commonly used. By way of illustration, FIG. 2 shows a curve 22 illustrating the temperature dependence of the carrier frequency f.sub.R supplied by such an AT crystal. As FIG. 2 shows, a temperature variation .DELTA.T causes a frequency variation .DELTA.f.sub.R to the AT crystal, in accordance with a polynomial type function of the order 3 with a linear term and a cubic term. Thus, within a temperature range comprised between 10 and 50.degree. C., the AT crystal can supply frequency f.sub.R substantially equal to 15 MHz, with a precision of the order of 3 KHz (which corresponds to the aforementioned precision of 2 to 10 ppm).
One drawback of using such an AT crystal is that the latter is expensive and has to be supplied at high frequencies, which involves high electric power consumption.
In the event that the generator forms part of a clockwork system including a time base formed by a CT crystal, one drawback of using an AT crystal is that the clockwork system fitted therewith includes two crystals whose low frequency signal supply is redundant. In this case, it would be preferable to use the CT crystal of the time base to form the reference oscillator of the generator, which would allow the number of components of the generator fitted to such a system to be reduced.
However, the fact of using the CT crystal of a clockwork system to form the reference oscillator of a high frequency signal generator has the drawback that the use of the very low frequency of this crystal involves high phase noise over the voltage controlled oscillator frequency spectrum.
Another drawback of such use of the CT crystal is that the frequency precision of this crystal is of the order of 60 ppm (10.sup.-6), which means that the frequency varies from 30 KHz over a temperature range of 60.degree. C. centred on 25.degree. C., which corresponds to a variation ten times greater than the aforecited precision of an AT crystal (2 to 10 ppm). This is detrimental in the event that the high frequency signal generator is used for transferring data to a transmission channel, as was mentioned hereinbefore.